The carcinogenic liver fluke Opisthorchis viverrini is a food-borne parasite that infects approximately 10 million people, primarily in low-income populations in Southeast Asia. People become infected when they consume raw or undercooked freshwater fish harboring O. viverrini metacercariae and infection can cause cholangiocarcinoma. The proposed research will characterize transmission pathways of O. viverrini, focusing on the role of fish movement and food-distribution networks in distributing O. viverrini parasites to human populations. The research will take place in and around three connected lakes in Northeast Thailand, an area with the highest incidence of O. viverrini-induced cholangiocarcinoma in the world. Using neutral genetic markers, we will assess the population structure of the parasite in the study region in order to assess whether there exist spatially distinct foci of transmission (e.g. transmission is constrained at the scale of a lake), r if there is widespread gene flow throughout the region, suggesting parasites are readily transported between lakes and villages. We will characterize harvested fish distribution networks. Using a smartphone-based exposure assessment protocol targeting fishermen, we will quantify connections between source lakes and villages throughout the study region in order to evaluate whether these connections constitute a regional transmission network. In addition, we evaluate the potential role of fish movement in distributing parasites- O. viverrini infections n the first intermediate snail host are rare (typically, prevalence is <2%) but fish infections are common (exceeding 90% prevalence in some areas), which suggests that fish may collect parasites from a patchwork of infected snails, distributing infections across their range. We will examine the distribution of O. viverrini infections in fish, sampling fish from different sites witin and between lakes in order to test the hypothesis that O. viverrini infections vary by lake, which, if detected, implies cyprinid host movements are constrained by lake geometry. We will ultimately evaluate the concordance of O. viverrini population genetic structure with the spatial patterns of food-distribution networks and fish infections, allowing us to infer key transmission pathways that act to constrain or facilitate the spread of O. viverrini. The proposed research will fill in crucial gaps in our understanding of this neglected tropical disease that can be used to guide infection prevention programs and food safety monitoring for O. viverrini and the related parasite, Clonorchis sinensis, which infects an estimated 35 million people. More generally, this disease system provides an opportunity to evaluate the role of food distribution systems and natural landscape boundaries in defining the spatial scale of infectious disease transmission.